Abstract: In order to guarantee the accuracy of measurements the observations mast be checked.In practice it is required that all hinds of discrepancy should not exceed their respective limiting values.But in the current official specification,the numbers of observation are not taken into account for defining the limiting values of discrepancy,this is not reasonable.So in this paper the author derived and calculated the probability distribution of extre me range by means of the principle of mathematical statisties,and put forward some requirements for checking the observations in the field works.